Composite structures may be fabricated by laying up a number of plies comprising woven or knitted fabric pre-impregnated with a carrier, such as a synthetic resin. During hand layup, successive plies are laid up by hand over a tool, and the fabric may be swept by hand in order to reduce possible wrinkling and bridging of the fabric. In some cases, where unacceptable wrinkling or bridging may occur in spite of sweeping, the plies must be lifted and reapplied in order to reduce the wrinkling/bridging. Plies may also be lifted and reapplied to shift the ply to meet ply placement tolerances.
To reduce dependency on hand layup and increase manufacturing flow rate, automated techniques have been devised for controlled placing of the plies during layup which substantially avoids wrinkling and bridging, and thus eliminates the need for rework. For example, automated equipment and methods for placing plies are disclosed in U.S. Pat. Nos. 7,004,219 and 7,213,629, and US Patent Publication number US 2006/0260751 A1, all owned by The Boeing Company, the entire disclosures of which are incorporated by reference herein.
U.S. Pat. No. 7,213,629 referred to above discloses a vacuum assisted ply placement shoe that may be used to place plies on a substantially flat substrate. The shoe includes a pair of straight edge seals respectively engaging the substrate and the ply to form an enclosed area in which a partial vacuum may be drawn. The partial vacuum draws the ply onto the substrate, and in combination with the ply seal, essentially eliminates wrinkling and bridging of the ply. While this placement shoe provides satisfactory results when used to place plies on flat substrates, it may not be effective for use with curved substrates because of the difficulty in maintaining a vacuum between the ply and curved substrate. In addition, wrinkling and/or bridging of the ply may occur because the ply is fed across a straight edge seal onto the curved substrate.
Accordingly, there is a need for a method and apparatus for placing plies on curved substrates that employs vacuum assisted ply placement with high accuracy while eliminating wrinkles or bridging of the ply. Embodiments of the disclosure are intended to satisfy this need.